Bottles obtained by biaxially stretching a saturated polyester such as polyethylene terephthalate (PET bottles) have excellent transparency, mechanical strength, heat resistance and gas barrier properties, and hence they have been widely used as containers for beverages such as juices, soft drinks and carbonated beverages.
For manufacturing bottles from such polyester, the polyester is generally injection molded into preforms which are then biaxially stretched by, for example, blow molding, to be shaped into bottles.
If acetaldehyde remains in the bottles thus obtained from the polyester, tastes of the contents filled in the bottles are markedly deteriorated. For this reason, the acetaldehyde content in the bottle-forming polyester is desired to be as low as possible. In order to reduce the amount of acetaldehyde contained in the bottle-forming polyester, it is required that the polyester not only has a low acetaldehyde content before molding but also hardly causes increase in acetaldehyde during molding.
Polyesters have conventionally been prepared by esterification reaction of a dicarboxylic acid such as terephthalic acid or its ester derivative and a diol such as ethylene glycol or its ester derivative and then polycondensation reaction of the resulting esterified products in the presence of a polycondensation catalyst. The polycondensation reaction comprises liquid phase and solid phase polycondensation steps, wherein a polyester from the liquid phase polycondensation step undergoes the solid phase polycondensation step to obtain a polyester having a high intrinsic viscosity and a low content of acetaldehyde.
Thus, conventional processes for preparing polyesters need both the liquid phase and solid phase polycondensation steps, the latter step taking a long period of time, whereby leading to increased production costs.
If a polyester obtained by liquid phase polycondensation without solid phase polycondensation is directly molded into an article such as a bottle, there is involved a problem that acetaldehyde is produced in the molding process, which may remain in the polyester molded article.
Accordingly, it has been desired to develop a process for preparing a polyester, in which a polyester having a low acetaldehyde content and hardly causing increase in acetaldehyde during molding into articles such as bottles can be prepared and the solid phase polycondensation step can be omitted.